Tamron's recent lenses are all fairly nice for the price, so sure, why not make a competitive 100-400. The older Canon was never that good and the mark II ramped the price up and, unsurprisingly, hasn't set the world on fire as a result. If Sigma and Tamron want to come in and take on that roughly-£1000 price tier which Canon has abandoned, sure, go for it. Here's a real easy prediction: it'll be better than the Canon mk I, worse than the mk II, and the only people who will really complain about it will be the ones who haven't used it. (Just like every other Sigma, Tamron, and Samyang lens.)
As for Canon, 200-500 is the way to go in terms of making a competitive product. Right now, thanks to their 200-500, D500 and now D850, wildlife and open field/long-distance sports photographers are switching, in droves, to Nikon. As someone with more serious (as in 'National Geographic and the BBC fight for their time' serious) wildlife photographers in my address book than I care to count, it's been really shocking to see how many have switched over in the last 12 months. To the best of my recollection at this moment, I only know two pros who haven't yet switched, and for one of those I know it's only until the 7D3 comes out and then they'll be making their decision based on the spec of that. Less verifiably, conversations I've had recently with several highstreet store managers and a couple of Sigma reps also suggest (but it's third-hand reporting) there's been a huge swing from Canon to Nikon in terms of these kinds of lenses and the bodies they're mostly used with. Yes, Canon is still the #1 for every beginner, most product photographers and most wedding/event photographers, but the wildlife and sports markets are being way better served by Nikon right now.
Myself, as a product pro who happens to dabble in wildlife as a hobby, even I'm eyeing up those Nikons and thinking it might be worth the switch.
It's already too late for many—Canon really should not have left it this long for the 7D3—but all is not entirely lost. Of course for shorter-range sports, Canon is still dominating the market with the 1DX2, and a used 7D and used 400mm f/5.6 or 100-400(mk I) is still the go-to for beginners. But everything between those two extremes is slipping away. If Canon wants to keep what remains of that vast middle portion of the sports/wildlife market, they need a combination which can at least match the Nikon combos. A 7D3 with no low-pass filter and a 200-500 f/5.6 IS at a good price (remember, that Nikon zoom costs the same as the Canon 400mm prime in most countries, yet is undeniably better in every single regard; Canon have been using the reputation of their big white lenses to price gouge for years) could be enough, assuming they're built well of course.
Going for 150-600 just means lower image quality, bigger bulk, and a higher price. At best you could eliminate one of those aspects by increasing the other two; image quality can be upped at the cost of size and price, size can be brought down at the cost of image quality and price, or price can be brought down at the cost of image quality and size. You don't get to have a super fun time with all three. That focal length simply can not be the quality, size, and price all at once that Canon needs in order to be competitive with Nikon's current golden child.
Of course, if the 7D3 turns out to have a low-pass filter then it's all irrelevant anyway. That'll be Canon willfully giving away the entire sports and wildlife markets other than the absolute beginners and the few indoor pros.
